feat: move Vanir signature generation to a cron job

[cuixq]

#4611

This PR introduces a new vanir_signatures worker to enrich OSV records with Vanir signatures. This cron job identifies modified vulnerabilities, generates the signatures, and updates both the Datastore and GCS with the enriched data.

• Implements the core logic for the signature generation cron job:
  • Tracks the last run time in Datastore to only process vulnerabilities modified since the previous execution.
  • Only processes vulnerabilities with at least one GIT affected range.
  • Skips vulnerabilities that already contain Vanir signatures in their database_specific field.
  • Also skips Linux Kernel vulnerabilities.
• Uses the vanir library to generate signatures and updates the database_specific field of affected packages.
  • Includes a --dry-run flag and --output-dir option for testing and inspection of generated signatures without modifying production data.

[cuixq]

/gemini review

[cuixq]

/gemini review